This application discloses an invention which is related, generally and in various embodiments, to a method for producing a magnetic resonance image using an ultra-short echo time.
Magnetic resonance imaging (MRI) is commonly used to provide detailed images of an object (e.g., a human body). However, for objects having short T2 relaxation times (e.g., ≦10 ms), images of such objects can not be properly produced by conventional MRI techniques which utilize pulse sequences with long echo times (e.g., >10 ms). Objects in the human body that have short T2 relaxation times include certain tissues and superparamagnetically-labeled therapeutic cells such as, for example, cartilage, knee menisci, ligaments, tendons, cortical bone, muscles, etc.
Ultra-short echo time magnetic resonance imaging (UTE-MRI) is beginning to attract more interest due to its potential to produce images of such tissues or cells in a non-invasive manner. UTE-MRI typically employs a specialized data acquisition technique to perform MRI scans in which very short echo times (e.g., <0.5 ms) are used. The echo time (TE) is usually defined as the time period from the center (or equivalent center) of an excitation pulse to the data acquisition at the k-space center. Both short excitation pulse (˜0.4 ms) and short data acquisition delay (<0.2 ms) after the excitation are pursued in UTE-MRI.
Three data acquisition techniques have traditionally been utilized with MRI to produce images of objects with short T2 relaxation times. In a first technique, a split sinc pulse is used to perform two-dimensional (2D) imaging. A first half-pulse is applied with a slice-select gradient (without a refocusing lobe) and a second half-pulse is employed with an opposite sign slice-select gradient. The data from both half-excitations, which are typically collected along radial trajectories, are combined to form a full slice-select acquisition. With this technique, no refocusing lobe is required. Data acquisition can begin almost immediately (limited by the shut down time of the radio frequency (RF) hardware) after the excitation. A drawback of this technique is that split excitations may introduce artifacts caused by bulk and/or physiological motion between the two individual excitations.
In a second technique, a three-dimensional (3D) excitation using a hard pulse (i.e., a rectangular pulse) is used to image a volume instead of a slice. This technique avoids many of the problems associated with slice selections. The hard pulse excitation offers an almost immediate (limited only by hardware) data acquisition and a short excitation as well. The hard pulse has the shortest duration for a flip angle in all possible RF pulses if the amplitudes of the pulses are the same.
Without slice-select gradients, however, the hard pulse excites the entire portion of the target object within the transmit coils instead of just a selected area/volume within the coils. This leads to a field-of-view (FOV) that is passively defined by the sensitivity maps of the receiver coils. In that circumstance, a volume is imaged and a 3D radial projection imaging (PI) trajectory is usually used to collect data in k-space.
Although this technique has short readout times (˜1 ms) and thus reduced signal loss, the 3D PI sampling requires a large number of radial projections in order to meet the Nyquist sampling requirement. A typical number of projections for a spatial resolution of 2 mm is approximately 31,000 which leads to a total acquisition time of about 26 minutes. The number of required projections may be reduced by partially twisting the radial trajectory. For example, by twisting each projection by 60%, the number of projections required for a spatial resolution of 2 mm is reduced by 60%, thereby decreasing the total acquisition time to about 10 minutes. The tradeoff for fewer projections in the twisted projection imaging (TPI) is the long projection arms used to maintain Nyquist sampling. A long projection arm means a long readout time (˜40 ms) due to the requirement of high slew rate for efficient TPI trajectory designs. Thus, the second technique also has some drawbacks.
In a third technique, a user-defined slice is targeted by using a selective excitation pulse (e.g., a sine pulse) and a slice-select gradient. This technique necessarily imposes a delay on data acquisitions due to its refocusing lobe. To avoid further delay of data acquisitions, variable-duration phase encodings are employed in the slice plane instead of fixed-duration phase encodings as used in most pulse sequences. The start time of data acquisition differs from one phase encoding to another and depends upon the duration of that particular phase encoding gradient. This technique results in an acquisition-weighted data collection mode, and is mainly utilized in MR microscopy and spectroscopic imaging (or chemical shift imaging) with ultra-short echo times.
Each of the above-described techniques is characterized by one or more limitations that make the techniques less than optimally suitable for imaging objects with short T2 relaxation times.